1. Field of the Invention
The present invention relates to a technique to assure that digital data is original.
2. Description of the Related Art
A technique is known to tamper with digital image data taken by a digital camera by using a commercially available tool such as a photo retouching tool. This leads to a reduction in credibility of digital image data compared with film photography. Thus, digital image data is low in admissibility of evidence. To resolve the above problem, U.S. Pat. No. 5,499,294 discloses a technique in which secret information is uniquely assigned to a digital camera and this information is embedded in the digital camera. If an image is taken by the digital camera and image data of the image is produced, then in the digital camera a digital signature is attached to the image data using the secret information whereby it becomes possible to ensure that the image data of the taken image is the original data by verifying the image data using the signature information attached thereto.
Image data of an image taken by a digital camera generally has a characteristic that the illumination in a peripheral area of the image is lower than that in a central area due to an optical characteristic of a lens. That is, the illumination gradually decreases from the center of the image toward the edge of the image. This phenomenon is called peripheral illumination falloff. To deal with the above situation, it is known to perform image processing to correct the reduction in illumination in the peripheral area of the image data. Hereinafter, this correction process will be referred to as a peripheral illumination correction process. More specifically, the ratio of the reduction in illumination in the peripheral area with respect to the illumination at the center is measured in advance, and pixel values are recalculated for the image data of the taken image based on the measured reduction ratio thereby reproducing correct pixel values.
However, in the technique disclosed in U.S. Pat. No. 5,499,294, the peripheral illumination correction is not taken into account. Therefore, if the peripheral illumination correction is performed for image data having a digital signature produced by a digital camera, it is determined in verification that the image data has been tampered with. If the peripheral illumination correction is not performed to prevent the image data from being determined as being tampered with, it is difficult to reproduce correct pixel values for the image data. That is, in the conventional technique, it is difficult to assure that image data is original while the image data is corrected not such that all pixel values are corrected by the same correction factor but such that each pixel is corrected differently to compensate for an inevitable change in pixel values. In particular, it is difficult to perform the peripheral illumination correction while assuring that image data is original. It is also difficult to perform the exposure correction while assuring that image data is original.